High durability heart valve

ABSTRACT

An improved heart bioprosthetic device having a metal frame wireform or stent having an outer external surface. The metal frame has a bond layer coating at least a portion of the external surface and a coating layer disposed on at least a portion of the bond layer. The bond layer comprises a metal selected from the group consisting of: chromium, titanium, zirconium, aluminum, platinum, palladium, and niobium. The coating layer is selected from the group consisting of: a metal nitride, a metal oxide, a metal carbide, and combinations thereof. The coating layer may have a thickness of about 10 μm or less and a grain size of about 10 nm to about 15 nm, and may be characterized as polycrystalline with randomly-oriented grains with both cubic and orthorhombic phases. In one embodiment, the bond layer comprises chromium and the coating layer comprises chromium nitride.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of Ser. No. 14/677,746, filed on Apr.2, 2015, which claims the benefit of U.S. Patent Application No.61/974,943, filed Apr. 3, 2014, the entire disclosure of which isincorporated by reference in its entirety.

TECHNICAL FIELD

The invention relates to a method of treating a metal substrate toincrease its durability and, more particularly, to a method of coatingheart valve frames, such as wireforms and stents, to increase thefatigue life.

BACKGROUND

Heart valves are dynamic structures that experience constant and cyclicmechanical stress from the hemodynamic forces intrinsic to its function.When the function of a natural heart valve declines or fails,replacement is typically required with a bioprosthetic heart valve.

One common type of bioprosthetic heart valve is a biological tissuevalve, which is usually coupled to and supported by a metal frame. Themetal frame can be either a wireform or a collapsible/expandable stent.Once implanted, the bioprosthetic heart valve is subjected to cyclichemodynamic forces, causing the leaflets to open and coapt. Theseforces, in turn, impart mechanical stresses onto the supporting metalframe. It is therefore desirable for the metal frame to have astructural integrity that is capable of withstanding these stresses.

The surface of a metal frame, such as a wireform or stent, may often beriddled with small imperfections that can ultimately lead to reducedfatigue life and premature failure. These flaws can be in the form ofinclusions (particles), draw lines, knit lines, or scratches, which areintroduced during the manufacture of the wire or tube used to form thewireform or stent, respectively. It is therefore desirable to remove orameliorate these imperfections before incorporating the metal frame intoa heart valve.

One method of addressing the surface imperfections of the metal frame isto mechanically polish the surface. It is difficult, however, tomechanically polish the surface of a metal frame because the surface isnot flat and typically has intricate or curved geometric configurations.It would be prohibitively difficult to uniformly polish the surface ofthe shaped metal frame. A similar challenge is presented with respect toelectropolishing. Additionally, while mechanical polishing orelectropolishing may remove certain imperfections, they may exposecertain other imperfections existing below the surface of the metalframe.

What is therefore needed is a method for treating a metal frame of abioprosthetic heart valve to improve its fatigue life, and thus,durability once implanted in a patient.

BRIEF SUMMARY

Methods and bioprosthetic heart valves comprising metal frames aredisclosed in which the metal frames may be subjected to furthertreatment to coat at least a portion of, if not the entirety of, theexternal surface with a bond layer and a coating layer, to therebyincrease its fatigue life and durability once implanted in a patient.The methods described herein are particularly advantageous in allowingfor a uniform application of a coating layer despite the curved,rounded, or otherwise intricate geometries of the metal frames thatconstitute a bioprosthetic heart valve. Moreover, process parameters forthe application of the bond and coating layers may be tailored so as tonot disturb the properties or the shape of the metal frame.

In one embodiment, a method for improving the fatigue life of a metalsubstrate is described. The method may comprise providing a metal frame,such as a stent or a wireform. The method may further comprise applyinga bond layer to at least a portion of an external surface of the metalframe. The method may further comprise applying a coating material to atleast a portion of the bond layer disposed on the external surface ofthe metal frame using a technique selected from the group consisting of:physical vapor deposition (PVD) and chemical vapor deposition (CVD). Thecoating may be applied at a temperature of about 150° C. (about 300° F.)or less. The coating layer may have a thickness of 10 μm or less.

In accordance with a first separate aspect, the metal frame may be madeof a material selected from the group consisting of: a metal alloy, ashape-memory metal and a super-elastic metal.

In accordance with a second separate aspect, the PVD may be alow-temperature arc-vapor deposition (LTAVD).

In accordance with a third separate aspect, the coating may be appliedat a temperature of about 145° C. (about 296° F.).

In accordance with a fourth separate aspect, the bond layer may compriseone or a combination selected from the group consisting of: chromium,titanium, zirconium, aluminum, platinum, palladium, and niobium.

In accordance with a fifth separate aspect, the bond layer and thecoating material may comprise the same metal.

In accordance with a sixth separate aspect, the coating material may bemade of one or a combination of materials selected from the groupconsisting of: a metal oxide, a metal nitride, and a metal carbide.

In accordance with a seventh separate aspect, the metal of the metaloxide, the metal nitride, or the metal carbide may be one or moreselected from the group consisting of: chromium, titanium, zirconium,aluminum, platinum, palladium, and niobium.

In accordance with an eighth separate aspect, the coating material maybe made of chromium nitride.

In accordance with a ninth separate aspect, the coating with thechromium nitride may be performed using LTAVD.

In accordance with a tenth separate aspect, the coating layer may have athickness of about 5 μm or less. The coating layer may have a thicknessabout 1 μm or less.

In another embodiment, an improved bioprosthetic heart valve isprovided. The bioprosthetic heart valve may comprise a metal frame and abiological tissue coupled to the metal frame forming leaflets of theheart valve. The metal frame may have an external surface and a bondlayer coating at least a portion of the external surface of the metalframe. The bond layer may comprise a metal selected from the groupconsisting of: chromium, titanium, zirconium, aluminum, platinum,palladium, and niobium. A coating layer may be disposed on at least aportion of the bond layer. The coating layer may be selected from thegroup consisting of: a metal nitride, a metal oxide, a metal carbide,and combinations thereof.

In accordance with a first separate aspect, the coating layer may have athickness of about 10 μm or less.

In accordance with a second separate aspect, the coating layer may havea grain size of about 20 nm or less.

In accordance with a third separate aspect, the coating layer may have agrain size of from about 10 nm to about 15 nm.

In accordance with a fourth separate aspect, the coating layer may haveboth cubic and orthorhombic phases.

In accordance with a fifth separate aspect, the coating layer may bepolycrystalline with randomly oriented grains.

In accordance with a sixth separate aspect, the bond layer and thecoating layer may comprise the same metal.

In accordance with a seventh separate aspect, the bond layer maycomprise chromium.

In accordance with a eighth separate aspect, the coating layer maycomprise chromium nitride.

Another embodiment provides a method for improving a fatigue life of ametal frame of an implantable device, the method comprising: disposing abond layer over at least a portion of a metal frame of an implantabledevice, the bond layer comprising at least one elemental metal; andvacuum depositing a coating layer over at least a portion of the bondlayer, the coating layer comprising at least one of a metal oxide, ametal nitride, or a metal carbide.

In some embodiments, disposing the bond layer comprises disposing a bondlayer by at least one of vacuum deposition or by electrochemicaldeposition. In some embodiments, the at least one elemental metal isselected from the group consisting of aluminum, titanium, zirconium,hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten,ruthenium, cobalt, rhenium, iridium, palladium, platinum, copper,silver, and gold. In some embodiments, disposing the bond layer over atleast the portion of the metal frame comprises disposing the bond layerover at least a portion of at least one of a stent or a wireform of aheart valve. In some embodiments, disposing the bond layer over at leastthe portion of the metal frame comprises disposing the bond layer overat least a portion of a metal frame comprising at least one of stainlesssteel, cobalt-chromium, titanium alloy, nitinol, a metal alloy, ashape-memory metal, or a super-elastic metal.

In some embodiments, vacuum depositing the coating layer comprisesvacuum depositing a coating layer using at least one of physical vapordeposition (PVD), chemical vapor deposition (CVD), or low-temperaturearc-vapor deposition (LTAVD). In some embodiments, vacuum depositing thecoating layer comprises vacuum depositing the coating layer at atemperature of about 150° C. (about 300° F.) or lower. In someembodiments, vacuum depositing the coating layer comprises vacuumdepositing a coating layer with a thickness of about 10 μm or less. Insome embodiments, the at least one of the metal oxide, the metalnitride, or the metal carbide of the coating layer comprises at leastone metal selected from the group consisting of aluminum, titanium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum,tungsten, ruthenium, cobalt, rhenium, iridium, palladium, and platinum.

In some embodiments, vacuum depositing the coating layer comprisesvacuum depositing a coating layer comprising chromium nitride. In someembodiments, the bond layer and the coating layer include the samemetal.

Another embodiment provides a prosthetic heart valve comprising: a metalframe; a bond layer disposed over at least a portion of the metal frame,the bond layer comprising at least one elemental metal; a coating layerdisposed over at least a portion of the bond layer, the coating layercomprising at least one of a metal nitride, a metal oxide, or a metalcarbide; and a plurality of leaflets secured to the metal frame, theplurality of leaflets defining a one-way valve for blood flowtherethrough.

In some embodiments, the at least one elemental metal of the bond layerincludes at least one of aluminum, titanium, zirconium, hafnium,vanadium, niobium, tantalum, chromium, molybdenum, tungsten, ruthenium,cobalt, rhenium, iridium, palladium, platinum, copper, silver, and gold.

In some embodiments, the coating layer has a thickness of about 10 μm orless. In some embodiments, the coating layer has a grain size of about20 nm or less. In some embodiments, the coating layer includes bothcubic and orthorhombic phases. In some embodiments, the coating layer ispolycrystalline with randomly oriented grains.

In some embodiments, the at least one of the metal oxide, the metalnitride, or the metal carbide of the coating layer comprises at leastone metal selected from the group consisting of aluminum, titanium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum,tungsten, ruthenium, cobalt, rhenium, iridium, palladium, and platinum.In some embodiments, the bond layer comprises chromium and the coatinglayer comprises chromium nitride.

Another embodiment provides a method for manufacturing a prostheticheart valve, the method comprising: vacuum depositing a coating layerover at least a portion of a metal frame; and securing a plurality ofleaflets to the metal frame, the plurality of leaflets defining aone-way valve for blood flow therethrough.

In some embodiments, vacuum depositing the coating layer comprisesvacuum depositing the coating layer using at least one of physical vapordeposition (PVD), chemical vapor deposition (CVD), or low-temperaturearc-vapor deposition (LTAVD). In some embodiments, wherein vacuumdepositing the coating layer comprises vacuum depositing a coating layercomprising at least one of a metal oxide, a metal nitride, or a metalcarbide.

Some embodiments further comprise disposing a bond layer over at least aportion of the metal frame, the bond layer comprising at least oneelemental metal. In some embodiments, disposing the bond layercomprising at least one of vacuum depositing a bond layer orelectrochemically depositing a bond layer.

In some embodiments, securing the plurality of leaflets comprisessecuring a plurality of tissue leaflets.

It is understood that each one of the separate aspects described abovemay be optional, may be provided alone, or in combination with any otheraspects. Other objects, features, and advantages of the describedembodiments will become apparent to those skilled in the art from thefollowing detailed description. It is to be understood, however, thatthe detailed description and specific examples, while indicatingembodiments of the present invention, are given by way of illustrationand not limitation. Many changes and modifications within the scope ofthe present invention may be made without departing from the spiritthereof, and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the present disclosure are described hereinwith reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a wireform used in the construction ofbiological tissue heart valves.

FIG. 2 is a perspective view of a stented biological tissue valve.

FIG. 3 is a partial cross-sectional view of a metal wireform or stentshowing a flaw that is revealed on an external surface of the metalwireform or stent and inclusions and flaws beneath the external surface.

FIG. 4 is a partial cross-sectional view of the metal wireform or stentof FIG. 3 having a bond layer and coating provided on the externalsurface.

FIG. 5 shows the fatigue test results for heavy/light coated CrN wiresand control uncoated wires as a function of kilopound per square inch(ksi) and number of stress cycles.

Like numerals refer to like parts throughout the several views of thedrawings.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

Specific, non-limiting embodiments of the present invention will now bedescribed with reference to the drawings. It should be understood thatsuch embodiments are by way of example only and merely illustrative ofbut a small number of embodiments within the scope of the presentinvention. Various changes and modifications obvious to one skilled inthe art to which the present invention pertains are deemed to be withinthe spirit, scope and contemplation of the present invention as furtherdefined in the appended claims.

Embodiments of the structures and methods disclosed herein are usefulfor a wide variety of implantable devices, and in particular, a deviceor component that is susceptible to failure from fatigue, for example,one that experiences repeated and/or cyclical loading and unloading.Examples of suitable devices include prosthetic heart valves, stents,annuloplasty rings and bands, and orthopedic and dental implants. Otherexamples include pacemaker leads and other prosthetics and devices usedto treat the heart and/or lungs. The disclosure focuses on prostheticheart valves, including bioprosthetic heart valves, but the structuresand processes apply equally to these and other implantable devices andcomponents.

The bioprosthetic heart valves and the methods disclosed herein providemetal frames, such as wireforms or stents, which may comprise a bondlayer and coating layer disposed on at least a portion of the externalsurface so as to enhance the fatigue life thereof.

Fatigue generally refers to the weakening of a material caused byrepeatedly applied loads and the progressive and localized structuraldamage that can occur when a material is subjected to cyclic loading.Thus, fatigue occurs when a material is subjected to repeated loadingand unloading. If the loads are above a certain threshold, microscopiccracks may form and eventually a crack may reach a critical size whereit may propagate and cause the structure to fracture. Fatigue life issometimes defined as the number of stress cycles of a specifiedcharacter that a specimen sustains before failure of a specified natureoccurs.

While metal frames for bioprosthetic heart valves can take on a numberof different forms, the most common configurations are wireforms andtubular stents. Some prosthetic valve frames include both a wireform anda stent.

Metal frames of a bioprosthetic heart valve may be subjected to fatigueby the hemodynamic forces that act upon the heart valve afterimplantation. The metal frames are typically wireforms or stents thatmay be made of a metal alloy, a shape memory metal, or a super-elasticmetal. Examples of suitable metals for frames include steel (forexample, stainless steel), nickel-titanium alloys (for example,nitinol), cobalt-chromium alloys (for example, alloys of cobalt,chromium, nickel, iron, molybdenum, and manganese, including Elgiloy® orMP35N™ cobalt-chromium alloys (Elgiloy Specialty Metals, Elgin, Ill.)),and titanium alloys (for example, titanium 6-4). Stents can be laser cutor machined from metal tubes, while wireframes are typically made frommetal wire, although other manufacturing methods are also used, forexample, 3D-printing, stamping, forging, and the like.

FIG. 1 is a perspective view of one example of a wireform frame 10 usedin the construction of prosthetic heart valves. The wireform frame 10includes alternating and oppositely-directed cusps 11 and commissuretips 12. The commissure tips 12 lie in a plane on an imaginary circle 2about axis 1. Likewise, the apices of the arcuate cusps 11 lie in aplane on an imaginary circle 3 about axis 1. Gradual bends 14 definetransitions between the commissure tips 12 and the adjacent cusps 11. Acrimp 16 holds together the two free ends of the wire used to form thewireform 10. The crimp 16 is typically a short, tubular metallic memberthat is compressed about the free ends and holds them by friction. Itwill thus be understood that the relatively complex contours of thewireform 10 may be controlled to a high degree to result in the desiredthree-dimensional shape. Leaflets are attached to the wireform frame 10,defining a one-way valve for blood flow therethrough. The exampleillustrated FIG. 1 includes three commissure tips 12 and cusps, andconsequently, accepts three leaflets arranged in a tricuspidconfiguration. In a bioprosthetic valve, the leaflets are made fromtissue or biological material, for example, pericardium, including, forexample, bovine, porcine, ovine, equine, or kangaroo pericardium. Otherexamples use synthetic leaflets, for example, polymer and/or fabric.Other valves have composite leaflets including both tissue and syntheticmaterial.

FIG. 2 is a perspective view of one example of a stented biologicalheart valve 20 comprising a biological tissue leaflet structure 28coupled to a metal stent 26. The leaflet structure 28 and metal stent 26can be configured to be radially collapsible to a collapsed or crimpedstate for introduction into the body on a delivery catheter and radiallyexpandable to an expanded state for implanting the valve at a desiredlocation in the body. The valve 20 in the illustrated embodiment furthercomprises a flexible skirt 30 secured to the outer surface of theleaflet structure 28 and has a lower inflow end 22 and an upper outflowend 24. The skirt 30 can be secured to the inside of the stent 26 viasutures 32. Blood flows upward freely through the valve 20 but theflexible leaflet structure 28 closes to prevent reverse, downward flow.As with the embodiment illustrated in FIG. 1, the leaflets can also besynthetic or composite.

Metal frames for bioprosthetic heart valves, such as the ones depictedand described with respect to FIGS. 1 and 2, can be treated inaccordance with the methods of applying a bond layer and coating layerdescribed herein to increase their inherent fatigue life, and thus,providing increased longevity of the implanted valve.

FIG. 3 illustrates the various flaws or defects that can be found bothon the surface of and also within the metal wireform or stent that canlead to premature failure and reduced fatigue life. Typical flaws takethe form of inclusions (particles), draw lines, knit lines, or scratchesthat may be introduced to the metal during the manufacture of the wireor tube used in the manufacture the wireforms and stents, respectively.Some of the imperfections can be eliminated by electropolishing the wireor tube, but not all imperfections can be removed and, in some cases,electropolishing can create additional flaws or expose of otherimperfections previously disposed under the external surface of thearticle.

FIG. 4 depicts the surface of the metal wireform or stent having a layerof a hard, wear-resistant coating, e.g., a bond or base layer, and acoating layer in the illustrated embodiment. In some embodiments, thewear-resistant coating covers all or substantially all of the outersurface of the metal frame, while in others, only a portion of theframe, for example, parts, assemblies, or subassemblies that are mostsusceptible to fatigue.

Some embodiments of the wear-resistant coating do not include a bondlayer, while other embodiments include a partial bond layer, that is, abond layer underlying only a portion of the coating layer. In some casesthe bond layer is completely overlaid by the coating layer, while inothers, at least a portion of the bond layer remains exposed.

Without being bound by any theory, it is believed that large compressiveresidual stresses may be generated by the differences in thermalexpansion and stiffness between the metal frame and the coatings. Thislarge residual stress lowers the operating stress at the metal frame,instead operating or manifesting at the coating surface, which isgenerally at least about 3 to 4 times stronger than the substrate.

One or more vacuum deposition processes, for example, physical vapordeposition (PVD), low-temperature arc-vapor deposition (LTAVD), and/orchemical deposition (CVD), can be used to apply each of the bond andcoating layers independently onto the external surface of the wireformsand stents to reduce or minimize premature failure and enhance fatiguelife. The term “vacuum deposition process” refers generally todeposition processes that are performed under reduced pressure. Whilethe term includes processes that are performed under vacuum, that is,substantially absent any gas pressure, it also includes processesperformed in the presence of one or more gases and/or plasma, at apressure lower than atmospheric pressure.

Physical vapor deposition (PVD) refers to a variety of vacuum depositionmethods used to deposit thin films by the condensation of a vaporizedform of the desired film material onto various work piece surfaces. Thecoating method may involve physical processes, such as high-temperaturevacuum evaporation with subsequent condensation, or plasma sputterbombardment rather than involving a chemical reaction at the surface tobe coated.

Developments in PVD permit vapor-deposited coatings to be applied atrelatively lower temperatures. An example of such a technique, known aslow-temperature arc-vapor deposition (LTAVD), can be used to applymetals and other materials at low and even at near ambient temperature.Parts to be coated may be placed in a chamber and revolve around acathode that serves as the metallic source of the coating. A vacuum isdrawn on the chamber and a low-voltage arc can be established on themetal source. The arc may evaporate the metal from the source.

The chamber may be charged with at least one or a mixture of inert andreactive gasses, such as argon, helium, and nitrogen, which may form anarc-generated plasma surrounding the source. Arc-evaporated metal atomsand reactive-gas molecules may ionize in the plasma and accelerate awayfrom the source. Arc-generated plasmas are unique in that they maygenerate a flux of atoms and molecules that have high energies and aremostly (>95%) ionized. The high energy may cause hard and adherentcoatings to form on the work piece mounted to one or more fixturesrotating around the source. A bias power supply may be used to apply anegative charge to the parts, which further boosts the energy of thecondensing atoms.

Chemical vapor deposition (CVD) is a chemical process that may be usedto produce high-purity, high-performance solid materials. In typicalCVD, the work piece is exposed to one or more volatile precursors, whichreact and/or decompose on the substrate surface to produce the desiredlayer, film, or deposit. Any volatile by-products may be removed by agas flow or purge through the reaction chamber.

In one embodiment, LTAVD may be utilized to apply both the bond andcoating layers onto the metal frame. The use of LTAVD may beadvantageous for wireforms and stents as some metals used for the metalframes (e.g., Elgiloy® cobalt-chromium alloy and nitinol) can besensitive to, and can change properties, when exposed to hightemperatures. In particular, higher temperatures can cause a frame tolose temper. A nitinol frame can lose its shape memory at highertemperatures as well. An acceptable temperature that can be used indeposit a coating layer onto any particular frame will depend on factorsincluding the particular composition of the frame, the thermal historyof the frame, and/or whether the frame was mechanically or workhardened. For example, in some embodiments the bond and/or coatinglayers may be applied at a temperature of about 150° C. (about 300° F.)or less, for example, at a temperature of about 145° C. (about 296° F.).Other frames can withstand deposition temperatures up to about 595° C.(about 1100° F.), while in others, the deposition is performed at about200° C. (about 400° F.) or lower. In some cases, lower temperatures areused with nitinol frames that have been shape-set.

Deposition rates may be from about 0.7 μm to about 1 μm per hour foreach layer. The coating layer may have a thickness of about 10 μm orless, a thickness of about 5 μm or less, or a thickness of about 1 μm orless. In some embodiments, the combined bond and coating layers,together, may have a thickness of about 10 μm or less, a thickness ofabout 5 μm or less, or a thickness of about 1 μm or less.

The deposition of the bond layer by LTAVD in a vacuum chamber may beperformed using an inert gas, such as argon or helium. In anotherembodiment, at least a portion of the bond or base layer is disposedonto the frame by a different method, for example, electrochemically.Embodiments of electrochemical depositions of the bond layer areperformed at from about 0° C. to about 100° C., for example, at aboutambient temperature. Moreover, however the bond and coating layers areapplied, each may independently be subjected to post-applicationtreatment or processing, for example, thermal and/or chemicalprocessing. Chemical processing includes contacting the coating layerwith one or more reactive chemical species, for example a gas, plasma,and/or liquid phase reactive species. Particular examples includereduction and oxidation, which can modify either a full or partialthickness of a coating layer.

The bond and coating layers may be applied to the metal frame after itis shaped and/or fabricated, but before it is assembled with thebiological tissue to form the final bioprosthetic heart valve. As themetal frame (e.g., the wireform or stent) has a three-dimensional,rounded, or cylindrical geometry, uniform application of the base andcoating layers may be achieved by rotating and moving either one or bothof the metal frame or metallic source relative to one another. In oneembodiment, the metal frame may be coupled to a movable support insidethe chamber that may rotate and expose substantially all sides of themetal frame to the plasma so as to provide a uniform coating of the baseand coating layers thereon. The support may couple to an area of themetal frame that experiences the least amount of stress or force. Forexample, for the wireform depicted in FIG. 1, the crimp 16 or thegradual bends 14 typically experiences the least amount of stress andthus may be an ideal location for coupling to the support. For the stent20 depicted in FIG. 2, this location may be the one of the verticalposts 40 or the apex 42 of the stent.

Without being bound by any theory, it is believe that the bond layer maypromote adhesion between the frame and the coating layer, and in someembodiments, may comprise any suitable material that is softer and morecompliant than the coating layer. As such, the bond layer can be a thinlayer, for example, as thin as from a few to a few tens of atoms thick.Thicker bond layers are used in some cases. Examples of thinner bondlayers have thicknesses of from about 3 Å to about 30 Å, or from about 5Å to about 15 Å. Embodiments of the bond layer are up to about 0.1 μm(100 Å) thick, for example, up to about 50 Å. The bond layer maycomprise a stable and non-reactive elemental metal including one or morenoble metals. The elemental metal of the bond layer may be biocompatibleor non-biocompatible. For example, in embodiments in which the coatinglayer completely covers the bond layer, no portion of the bond layer isexposed, and as such, biocompatibility is less important. As such,factors including deposition conditions, ease of deposition,reproducibility, compatibility with the coating layer, adhesion of thecoating layer, durability of the entire wear-resistant coating, andimprovement in fatigue resistance can take precedence in such cases.Some embodiments of the bond layer include a plurality of layers ofdifferent materials, for example, for improved lattice matching betweenthe underlying metal frame and the coating layer and/or to encapsulate aless biocompatible metal. Accordingly, the bond layer may comprise anyone or a combination of elemental metals selected from aluminum,titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium,molybdenum, tungsten, ruthenium, cobalt, rhenium, iridium, palladium,platinum, copper, silver, and gold.

The coating layer may comprise one or a combination of materialsselected from a metal oxide, a metal nitride, and a metal carbide. Inone embodiment, the metal of the metal oxide, the metal nitride, or themetal carbide may comprise one or more of aluminum, titanium, zirconium,hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten,ruthenium, cobalt, rhenium, iridium, palladium, and platinum. Thecoating layer may be polycrystalline with randomly-oriented grains. Thegrain size may be about 20 nm or less, or from about 10 nm to about 15nm. Additionally, the coating layer may further comprise both cubic andorthorhombic phases. Application of the coating layer by LTAVD in avacuum chamber may be performed with at least one reactive gas selectedfrom the group consisting of N₂, O₂, CO₂, and CH₄. Some embodiments usea gas mixture further including at least one inert gas, for example,argon or helium.

In some embodiments, the bond layer and the coating layer may comprisethe same metal, which may facilitate manufacture of the wear-resistantcoating. For example, after depositing a bond layer by LTAVD using aparticular metallic source and an inert gas to generate the plasma,adding an appropriate gas to the reaction chamber, for example,nitrogen, oxygen, or methane, permits depositing a coating layer of thesame metal nitride, oxide, or carbide, respectively.

In another embodiment, the bond layer may be chromium and the coatingmaterial may be chromium nitride (CrN). Chromium nitride coatingsexhibit corrosion resistance, as well as hardness and wear-resistance.

EXAMPLE

A bond layer of chromium was deposited onto Elgiloy® cobalt-chromiumwires by LTAVD under argon. After depositing a thin layer of chromium (afew Angstroms thick), nitrogen gas was introduced into the reactionchamber to deposit a coating layer of CrN at a rate of 0.7-1 μm/hr. Thetemperature was kept below 150° C. (300° F.) throughout the process. Ina heavy-coated set of wires, a 1.4-2 μm thick coating layer of CrN wasdeposited over the chromium bond layer. In a light-coated set of wires,the CrN coating layer was about 0.7 μm thick. FIG. 5 is a graph offatigue test results of the heavy-coated wires, the light-coated wires,and a control group of uncoated wires. Each of the wires was subjectedto 10 million cycles at a fixed mean stress and amplitude. If the wiredid not fracture, the stress amplitude was increased and the wire wassubjected to an additional 10 million cycles. The stress amplitude wasfurther increased after each set of 10 million cycles until the wirefractured. As demonstrated in FIG. 5, the uncoated wires fractured at alower number of cycles and at lower stress amplitudes as compared to thecoated wires, with the CrN heavy-coated wires generally fracturing aftera higher number of cycles and at higher stress amplitudes than the CrNlight-coated wires.

The invention described and claimed herein is not to be limited in scopeby the specific embodiments disclosed herein, as these embodiments areintended as illustrations of several aspects of the invention. Indeed,various modifications of the invention in addition to those shown anddescribed herein will become apparent to those skilled in the art fromthe foregoing description. Such modifications are also intended to fallwithin the scope of the appended claims.

What is claimed is:
 1. A prosthetic heart valve comprising: a metalframe; a bond layer disposed over at least a portion of the metal frame,the bond layer comprising at least one elemental metal; a coating layerdisposed over at least a portion of the bond layer, the coating layercomprising at least one of a metal nitride, a metal oxide, or a metalcarbide; and a plurality of flexible leaflets supported by the metalframe to form a one-way valve for blood flow therethrough.
 2. Theprosthetic heart valve of claim 1, wherein the metal frame comprises aradially collapsible and radially expandable stent or an undulatingwireform.
 3. The prosthetic heart valve of claim 1, wherein the metalframe comprises at least one of stainless steel, cobalt-chromium,titanium alloy, nitinol, a metal alloy, a shape-memory metal, or asuper-elastic metal.
 4. The prosthetic heart valve of claim 1, whereinthe at least one elemental metal of the bond layer includes at least oneof ruthenium, palladium, silver, iridium, platinum, and gold.
 5. Theprosthetic heart valve of claim 1, wherein the coating layer has athickness of about 10 μm or less.
 6. The prosthetic heart valve of claim1, wherein the coating layer has a grain size of about 20 nm or less. 7.The prosthetic heart valve of claim 1, wherein the coating layerincludes both cubic and orthorhombic phases.
 8. The prosthetic heartvalve of claim 1, wherein the coating layer is polycrystalline withrandomly oriented grains.
 9. The prosthetic heart valve of claim 1,wherein the at least one of the metal oxide, the metal nitride, or themetal carbide of the coating layer comprises at least one metal selectedfrom the group consisting of aluminum, titanium, zirconium, hafnium,vanadium, niobium, tantalum, chromium, molybdenum, tungsten, ruthenium,cobalt, rhenium, iridium, palladium, and platinum.
 10. The prostheticheart valve of claim 1, wherein the bond layer comprises chromium andthe coating layer comprises chromium nitride.
 11. A prosthetic heartvalve comprising: a metal frame; a bond layer disposed over at least aportion of the metal frame, the bond layer including one or more stableand non-reactive elemental noble metals; a coating layer disposed overat least a portion of the bond layer; and a plurality of flexibleleaflets supported by the metal frame to form a one-way valve for bloodflow therethrough.
 12. The prosthetic heart valve of claim 11, whereinthe metal frame comprises a radially collapsible and radially expandablestent or an undulating wireform.
 13. The prosthetic heart valve of claim11, wherein the metal frame comprises at least one of stainless steel,cobalt-chromium, titanium alloy, nitinol, a metal alloy, a shape-memorymetal, or a super-elastic metal.
 14. The prosthetic heart valve of claim11, wherein the one or more stable and non-reactive elemental noblemetals of the bond layer include at least one of aluminum, titanium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum,tungsten, ruthenium, cobalt, rhenium, iridium, palladium, platinum,copper, silver, and gold.
 15. The prosthetic heart valve of claim 11,wherein the bond layer comprises chromium and the coating layercomprises chromium nitride.
 16. The prosthetic heart valve of claim 11,wherein the coating layer comprises at least one of a metal nitride, ametal oxide, or a metal carbide.
 17. The prosthetic heart valve of claim11, wherein the coating layer has a thickness of about 10 μm or less.18. The prosthetic heart valve of claim 11, wherein the coating layerhas a grain size of about 20 nm or less.
 19. The prosthetic heart valveof claim 11, wherein the coating layer includes both cubic andorthorhombic phases.
 20. The prosthetic heart valve of claim 11, whereinthe coating layer is polycrystalline with randomly oriented grains.